An image forming apparatus includes a plurality of photoconductive drums for forming color images and a plurality of laser beam units which emit a laser beam, and forms a latent image on each of the photoconductive drums by performing the main scanning and the sub scanning on the photoconductive drums, using the laser beam emitted from these laser units. In addition the image forming apparatus develops the latent image formed on each of the photoconductive drums, and prints the image on an image forming medium, for example, on a paper sheet.
The image forming apparatus has a plurality of laser beam detection sensors. These laser beam detection sensors detect a laser beam which is emitted from each of the laser units, as the reference position of the main scanning with respect to each of the photoconductive drums. If something goes wrong with any one of these laser beam detection sensors, a problem occurs when forming images.